Here's a thought experiment: imagine that activists, concerned with official misconduct, install license-plate readers on private property to track the location of every car belonging to the police department or a politician and upload the locations to a public database. The result: a map of where the police go, and where they don't—along, perhaps, with politicians' visits to motels or strip clubs.

Given that police often respond with hostility to simply being videotaped, I expect that a venture like this would prompt an outcry, and probably some efforts to shut it down. But this is precisely what officialdom is doing to citizens.

We now know that federal, state, and local law enforcement agencies are using automated license-plate scanners, mounted on everything from telephone poles to police cars, to build a huge database of where people are driving. This might seem like a small intrusion compared with the electronic spying carried out by the NSA. But not all threats to privacy involve the tracking of emails and other communications.

Right now, the law suggests that license-plate scanners don't invade your privacy because they record only events that occur in public. After all, anyone could see you driving down the road or parked in front of a motel. But if officials add up enough bits of information like that, they gradually can construct what the ACLU has termed a "single, high-resolution image of our lives."

There's a legal term for this idea: the mosaic theory. The New York Timesran a story last year about how a man angrily confronted a Target store manager to complain that the company was sending his teenage daughter coupons for baby goods. Were they trying to encourage her to get pregnant? Nope. Target's data-mining operation had found a strong correlation between purchases of about 25 items—scent-free lotions, certain nutritional supplements, and so on—and different stages of pregnancy. The teenager's purchases had fit the pattern. The father apologized to Target a few days later, when it turned out that his daughter was, in fact, pregnant.

Law enforcement agencies may not know or care what toiletries you buy, but they can access credit reports and property tax records, which are public information. Setting that aside, simply tracking our movements can erode our privacy. The Supreme Court recently held that police need a search warrant to attach a GPS tracker to an individual's car, even though the device would just be recording travel along public roads. The decision turned largely on the idea that placing a locating device on your car is a trespass on your property. But five justices also suggested some sympathy to the mosaic theory as a legal argument; whether the court actually adopts such an approach will have to wait for a later case.

The Supreme Court, though, isn't the first step in protecting privacy; it's the last. If people are unhappy with the notion of having their movements, email, Web searches, and other behavior tracked, there's nothing to stop Congress, or state legislatures, from limiting this sort of activity, both on the part of private businesses and, more significantly, on the part of law enforcement. When it comes to protecting your rights and privacy, there's no reason to wait for courts to act.

And while we're at it, maybe we should enable the public to return the surveillance favor. In his prophetic 1998 book, The Transparent Society, David Brin wrote that technology was going to make it almost impossible to stop snooping. But, he suggested, if the government and corporations want to spy on us, they should let the public know what they are doing, too, by letting us track their data. We can set up Web feeds from every police headquarters, for a start. Today, we're living in Brin's world, or at least we're halfway there. Big organizations are already watching individuals—perhaps it's time to open things up in the other direction.